Generally, flexible circuit boards are light, soft, thin, small, ductile, flexible, and supporting high wiring density. Flexible circuit boards can be three-dimensionally wired and shaped according to space limitations. Flexible circuits are generally useful for electronic packages where flexibility, weight control and the like are important.
FIG. 11 illustrates a high frequency signal line structure 100 of a conventional flexible circuit board. The high frequency signal line structure 100 includes a dielectric layer 150, a signal line 110, a first grounding circuit layer 120, a second grounding circuit layer 130 and a plurality of conductive holes 140. The first grounding circuit layer 120, the second grounding circuit layer 130 and the conductive holes 140 collectively surround the signal line 110 and are layered in the dielectric layer 150. FIG. 12 and FIG. 13 illustrate that the second grounding circuit layer 130 includes a plurality of openings 131 and a plurality of bridges 132. The openings 131 and the bridges 132 are alternately arranged in the second grounding circuit layer 130 and expose portions of the signal line 110. The conductive holes 140 are formed at two opposite sides of the plurality of bridges 132 and electrically connect the first grounding circuit layer 120 and the second grounding circuit layer 130.
However, the high frequency signal line structure 100 still has a problem of serious signal loss when the signals are transmitted.